Timber construction systems for dwellings and furniture are simple, rapid and cost competitive compared to most other construction systems. Timber (also called lumber) is highly anisotropic, with excellent longitudinal strength but poor resistance to bending and impaction in transverse directions. Further shortcomings of solid timber, or lumber, are flammability, susceptibility to rot and insect damage, and deformation, the latter problems being exacerbated by water uptake.
Engineered wood products such as plywood or laminated veneer lumber (LVL), and reconstituted board products such as oriented strand board (OSB) and medium density fibreboard (MDF), are glued composites of wood veneers, strands and fibres respectively. Such products are designed to help overcome some of the shortcomings of timber.
A feature of engineered or reconstituted wood products is the alignment or re-orientation of veneers, strands or fibres, and the introduction of glues, resins and waxes, to improve properties such as strength, hardness and dimensional stability in order to match the performance requirements of specific applications. However, these modifications in turn introduce new problems such as creep in MDF.
To alleviate the shortcomings mentioned above there is a need to modify wood and further modify engineered and reconstituted board products, or their components, in order to improve physical properties such as fire retardancy, strength, surface hardness, dimensional stability, water resistance, UV resistance and durability with respect to rot and insect damage (biological resistance). At least some of these improvements may be linked.
These improvements in the properties of wood may be achieved in a number of ways including chemical treatment, heating and compression.
Conventional improvements have been restricted to improving the resistance to rot or insect attack by the inclusion of water borne or solvent borne chemical systems either by dipping, spraying or the application of vacuum and/or pressure. These systems generally rely on filling of the wood cell lumen and sometimes also the cell wall with the chemical system which is fixed partially or completely to the cellulose and hemicellulose of the wood.
In order to improve physical properties such as strength, hardness or dimensional stability it is necessary to bring about more significant changes. One instance is by the introduction of chemicals capable of derivitization, or polymerisation and/or crosslinking. For example Norimoto et al. (1992. Wood and Fiber Science 24, 25-35) teach that a chemical treatment such as this must result in filling of the cell lumen and/or modification of cell wall material. Ideally lumen modification will include good interfacial adhesion and cell wall modification will include covalent crosslinking of introduced materials to the cellulose, hemicellulose or lignin constituents.
Furthermore requirements are that the chemical modification retains the characteristic colour, workability and glueability of the native wood without excessive densification, that the method of treatment is compatible with manufacturing processes for engineered or reconstituted wood products, and that the treatment composition utilises materials that are cheap, abundant and sustainably produced.
Known in the prior art is the simple approach of impregnating wood or engineered wood products with one or more resins soluble in water or water-miscible solvents followed by polymerisation in situ, generally brought about by heat.
Gindl et al. (2004. Journal of Applied Polymer Science 93, 1900-1907) teach the use of aqueous 30% melamine-formaldehyde to increase the surface hardness of Norway spruce wood to that typical of hardwood beech. An important disadvantage is the complexity or duration of the treatments, i.e. vacuum impregnation repeated three times over 10 minutes or, more effective, a 3 day solvent exchange process applied to water saturated timber.
Lukowsky (2002. Holz als Roh- and Werkstoff 60, 349-355) demonstrated only modest improvements in dimensional stability and water resistance in Scots pine solid wood vacuum impregnated with melamine-formaldehyde resins. Dimensional stabilisation and cell wall penetration were inversely related to the molecular weight or degree of condensation of the resins.
US patent 2005/0170165 discloses full cell impregnation of softwoods with furfuryl alcohol monomer mixtures followed by heat curing. The treatment produces markedly increased resistance to microbial decay but only slightly increased hardness, bending strength and elasticity, and decreased impact resistance.
U.S. Pat. No. 7,008,984 discloses a related treatment process that also increases biological resistance but with excessively high densification (68%-80% weight gain) for many applications.
Magalhaes and Silva (2002. Journal of Applied Polymer Science 91, 1763-1769) teach that treatment with furfuryl alcohol, which is hydrophilic and compatible with polar wood macromolecules, provides good cell wall penetration, resulting in increased dimensional stability, whereas treatment with polystyrene, which is hydrophobic, fills the cell lumen and provides good cell adherence, producing significantly improved water repellency. Unfortunately both resins, when applied together or in sequence, polymerise poorly and do not produce the expected combined benefits of the two treatments.
The examples above show that infiltration and crosslinking of resins alone within wood does not provide the sort of benefits that are sought, particularly for plantation grown softwood species.
In an alternative approach a polymerising resin is combined with low molecular weight hydrophilic materials compatible with wood cell polymers.
NZ patent 235036 and U.S. Pat. No. 5,770,319 teach that treatment of radiata pine with a prepolymer formed from hexamethoxymethyl melamine and maltodextrins, followed by heating to form a polymer within the wood cell wall, results in an increase in modulus of elasticity (MoE) of about 12% at weight gains of approximately 40-60%, as well as increased hardness and biological resistance. Disadvantages of using the process disclosed include the production of excessively heavy wood and insufficient stiffening (12%) for many structural applications.
Franich et al. (2005. Proceedings 13th ISWFPC, pp. 73-79) disclose an improved composition formed from hexamethoxymethyl melamine and chitosan oligosaccharides that produces an increase in MoE of up to 20%. Disadvantages of the process disclosed include the prohibitive cost of the starting material, chitosan (currently around US$15/kg), and the need to depolymerise chitosan prior to polymerisation.
Franich et al. (op. cit.) teach that oligosaccharides used to form prepolymers must be small enough (<1,000 Da) to penetrate lignocellulosic cell walls.